The conventional adjustable wrench is generally provided with a fixed jaw and a movable jaw, which form together a clamping mouth for fastening or loosening a fastener such as a hexagonal nut or bolt head. Such a prior art adjustable wrench as described above is defective in design in that it often fails to hold a nut or bolt head firmly at the time when the nut or the bolt head is twisted strenuously, and that the movable jaw is vulnerable to a crack.